1. Field of the Invention
The present invention relates to a starting clutch which can be used in place of a torque converter for a motor vehicle and the like.
2. Description of the Related Art
In the past, in automatic transmissions, the starting of a vehicle has been achieved through torque transmission of a torque converter. The torque converter has been mounted on many vehicles since the torque converter serves to amplify the torque and also provides smooth torque transmission.
On the other hand, the torque converter has disadvantages that a slip amount is increased during the torque transmission and that efficiency is relatively bad.
Thus, in recent years, it has been proposed that a starting clutch is used in place of the torque converter and it has also been implemented that, in a low speed range, the torque is amplified by increasing a gear ratio and by increasing the number of transmission stages.
In general, the starting clutch includes a wet type multi-plate clutch housed in a clutch drum. In the multi-plate clutch, friction plates as friction engaging elements at an output side and separator plates as friction engaging elements at an input side are alternately arranged along an axial direction. With this arrangement, when the friction plates and the separator plates are engaged with each other by a piston, a power can be transmitted.
In the starting clutch, since great heat is generated, it is necessary to supply a large amount of lubricating oil for the cooling. Thus, although an oil pump must has a great pump capacity, a pump capacity of an oil pump provided in the conventional transmission is insufficient, and, therefore, there arises a problem that such an insufficient oil pump cannot be mounted to the starting clutch as it is. Further, since the conventional oil pumps perform the cooling operation with a small amount of oil, heat may be accumulated in the clutch portion and then the clutch may be burned.
Japanese Patent Application Laid-open No. 2002-357232 discloses a starting clutch in which a large amount of lubricating oil is used to cool the clutch and a plurality of holes is provided to discharge the lubricating oil along a radial direction of a clutch drum. In this case, however, since the lubricating oil is discharged faster from the clutch portion, heat exchange between the clutch portion and the oil cannot be achieved sufficiently, with the result that the cooling efficiency of the clutch may be worsened and the heat is apt to be accumulated in the clutch portion. Further, as described in U.S. Pat. No. 6,929,105, if the oil is filled, although the heat of the clutch portion is transmitted to the oil, the oil is retained in the clutch portion for a longer term, and, thus, similar to the above-mentioned Japanese Patent Application Laid-open No. 2002-357232, the heat may not be removed from the clutch portion smoothly.